In the packaging of certain types of foods, such as snack foods like potato chips, cookies and the like, it is common practice to employ a multi-layer film. A desirable property in such a packaging film is an opacity which protects the packaging product from deterioration caused by exposure to light. In particular. It has been found that certain wavelengths of light, up to about 450 nm cause increased spoilage in such packaged products. Even when a degree of opacity is present in the film, spoilage occurs if the film allows passage of some light.
It is known in the art that thermoplastic polymers can be loaded with inert fillers, cast into films, and thereafter stretched to form oriented thermoplastic films. While this statement is generally true, it must be realized that the particular components employed and the specific process parameters employed, particularly when control is difficult, can result in significantly different end products or contribute to the success or failure of obtaining a desired result. As an example, U.S. Pat. No. 4,118,438, discloses the use of some materials similar to those contemplated by the present invention, however, the object of U.S. Pat. No. 4,118,438 is diametrically opposed to the object of the present invention. U.S. Pat. No. 4,118,438 is concerned with a transparent polypropylene film containing fine particles of an incompatible polymer dispersed therein. The film disclosed exhibits surface projections caused by the dispersed particles and the patentees maintain that this gives the transparent film non-blocking characteristics. In U.S. Pat. Nos. 3,697,367 and 3,795,720, there is disclosed a process for preparing an uniaxially oriented mixed polymer system. The resulting material has utility as a paper substitute and can be formed into fibers for making sheet paper.
Oriented opaque film compositions are known in the art. U.S. Pat. No. 4,377,616 discloses an opaque biaxially oriented polymer film structure comprising a thermoplastic polymer matrix core possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent thermoplastic skin layers adhering to the surfaces of the core layer. The unique structure of the core layer imparts a much higher degree of opacity, possibly due to the effects of light scattering, than that possible from the use of an opacifying pigment alone. In U.S. Pat. No. 4,377,616, the film is prepared by melting a mixture of a major proportion of a film forming polymer such as polypropylene and a minor proportion of an incompatible polymer which has a higher melting point, at a temperature sufficient to melt the incompatible polymer and to dispense it in the film forming polymer, extruding the mixture into a film and biaxially orienting the film. The dispersed incompatible polymer provides sites for the formation of voids surrounding the polymer particles. These voids provide opacity and give the film an attractive pearlescent sheen.
U.S. Pat. No. 4,632,869 discloses an opaque, biaxially oriented film structure having a polymer matrix with a strata of voids, the voids containing spherical void-initiating particles of polybutylene terephthalate. The structure may also include thermoplastic skin layers and the film can include from about 1% to 3% b weight of a pigment such as TiO.sub.2 or colored oxides.
U.S. Pat. No. 4,758,462 also discloses an opaque, biaxially oriented film with a cavitated core and transparent skin layers. Colored light absorbing pigments such as carbon black or iron oxide are added to the core and/or the skins in an amount of about 2 to 12 weight % to decrease light transmission through the film.
U.S. Pat. No. 4,652,489 discloses an oriented, sealable, opaque polyolefin multi-layer film with a core layer containing vacuoles, a sealable surface layer, and a non-sealable surface layer which incorporates a slip agent such as a polydiorganosiloxane.
U.S. Pat. No. 4,741,950 discloses a differential opaque polymer film with a core layer containing numerous microscopic voids, a rough-appearing wettable first skin layer which contains an antiblocking agent such as silica, silicate, clay, diatomaceous earth, talc and glass, and a second wettable skin layer with a smooth appearance which may be metallized. TiO.sub.2 may be present in the core and/or first skin layer. The film allows a light transmission of 24%.
U.S. application Ser. No. 07/324,134, a co-inventor of which is also the inventor of the present invention, discloses a non-symmetrically layered, highly opaque, biaxially oriented polymer film with a core containing numerous microscopic voids and at least about 1% by weight of opacifying compounds; a first skin layer on one surface of the core containing up to about 12% by weight of inorganic particulate material; and a second skin layer on the other surface of the core. U.S. application Ser. No. 07,324,134, also discloses the benefit which accrues from the addition of inorganic particles such as titanium dioxide to whiten the surface of the outer skin layer of the film structure. The increase in whiteness yields an excellent surface for printed graphics. A further benefit resulting from increased whiteness in the outer skin layer of the film is that it permits the printing of laminated or unlaminated film structures without the need for while ink, offering a significant savings to the end user.
While films which employ titanium dioxide-whitened outer skin layers provide the aforementioned benefits, such films can also yield certain undesirable characteristics. These characteristics stem from the fact that titanium dioxide (TiO.sub.2) is quite abrasive and, when present on the surface of a film, may result in excessive wear of expensive printing and coating gravure roll surfaces, as well as any other surface which is contacted by such a film. Another problem which arises from the use of TiO.sub.2 in the outer skin layers of such films is that fine deposits are laid on converting machinery, extruder die lips, treater bar exhausts, etc. Also, appearance problems caused by streaks on the film slippage on stretching either by roll or tentering can result.
U.S. application Ser. No. 07/699,864, now U.S. Pat. No. 5,091,236, the inventors of which are also the inventors of the present invention, discloses multi-layer opaque, biaxially oriented polymeric film structures which avoid the problems associated with films employing titanium dioxide-whitened outer skin layers. The film structures disclosed include (a) a thermoplastic polymer matrix core layer having a first surface and a second surface, within which is located a strata of voids; positioned at least sbustantially within a substantial number of the voids is at least one spherical void-initiating particle which is phase distinct and incompatible with the matrix material, the void space occupied by the particle being substantially less than the volume of the void, with one generally cross-sectional dimension of the particle at least approximating a corresponding cross-sectional dimension of the void; the population of the voids in the core being such as to cause a significant degree of opacity; (b) at least one thermoplastic polymer intermediate layer having a first surface and a second surface, the second surface of the intermediate layer adhering to at least the first surface of the core layer, the intermediate layer including up to about 12% by weight of titanium dioxide contact pigment; and (c) a titanium dioxide-free, non-voided thermoplastic skin layer adhering to the first surface of the intermediate layer, the void-free skin layer and the intermediate layer together being of a thickness such that the outer surface of the skin core layer does not, at least substantially, manifest the surface irregularities of the matrix core layer. U.S. application Ser. No. 07/699,864 is hereby incorporated by reference in its entirety for all that it discloses.
Despite these significant advances in the art, many of the films described within the above-referenced patent literature prove to be somewhat fragile in certain end-use applications. While highly cavitated or voided films are ideally suited for certain applications, such as overwrapping biscuits and the like, in other applications such as those employing Vertical Form Fill and Seal (VFFS) packaging machinery, as well as some which employ Horizontal Form Fill and Seal (HFFS) machinery, the forming collars of the machine often damage or shear the package being formed and filled. In addition, films converted into packages which are filled with both product and air, so as to provide an air cushion for product protection, often benefit from the use of a less fragile film. However certain means employed to provide a less fragile voided film can reduce the film's pleasing aesthetic appearance or may change the ability of the film to inhibit the transmission of light therethrough.
Therefore, what is needed is a less fragile, opaque film structure which provides an improved range of machining operability, while maintaining its appearance characteristics, strength and stiffness and a process for the production of such a film structure.